Preoperative prediction of the pathological stage of advanced gastric cancer by 18F-fluoro-2-deoxyglucose positron emission tomography

In recent years, the usefulness of neoadjuvant chemotherapy for resectable advanced gastric cancer, particularly stage III, has been reported. Preoperative staging is mainly determined by computed tomography (CT), and the usefulness of 18F-fluoro-2-deoxyglucose positron emission tomography/CT (FDG-PET/CT) for gastric cancer has been limited in usefulness. The study aimed to evaluate the usefulness of FDG-PET/CT in preoperative diagnosis of advanced gastric cancer. We retrospectively enrolled 113 patients with gastric cancer who underwent preoperative FDG-PET/CT. All patients underwent gastrectomy with lymph-node dissection. The maximum standardized uptake value (SUVmax) of the primary tumor (T-SUVmax) and lymph nodes (N-SUVmax) were measured for all patients. The cutoff values of T-SUVmax for pathological T3/4 from receiver operating characteristic analysis were 8.28 for differentiated and 4.32 for undifferentiated types. The T-SUVmax and N-SUVmax cutoff values for pathological lymph-node metastasis were 4.32 and 1.82, respectively. Multivariate analysis showed that T-SUVmax for differentiated types was a significant predictor of pathological T3/4, and N-SUVmax was a significant predictor of lymph-node metastasis. In conclusion, the SUVmax of FDG-PET/CT was a useful predictor of pathological T3/4 and lymph-node metastasis in gastric cancer. The diagnosis by preoperative FDG-PET/CT is promising to contribute a more accurate staging of gastric cancer than by CT scan alone.


Discussion
The SUVmax in primary GC tumors has been shown to correlate with tumor invasion depth [12][13][14] , but few studies have predicted pT status by using a cutoff value. By analyzing the relationship between histological type and SUVmax, we found that the uptake of FDG in primary tumors of GC depended on histological type, with higher uptake in differentiated types and lower uptake in undifferentiated types. Therefore, in undifferentiated types, multivariate analysis did not show that the T-SUVmax was useful for diagnosis of pT3/4. This finding is consistent with those of several previous reports and explains the low utility of FDG-PET/CT in GC [15][16][17] . However, our results showed that the T-SUVmax was a significant predictor of pT3/4 diagnosis in differentiated types by using a cutoff value with the ROC curve.
In the diagnosis of lymph-node metastasis, Oh et al. have shown that T-SUVmax on FDG-PET/CT is a useful predictor of pathological lymph-node metastasis in GC 18 . These studies focused only on the metabolic activity of the primary tumor. We considered that using the metabolic activity of lymph nodes as a predictor of lymph-node metastasis would further improve the diagnostic accuracy. Tsunoda et al. revealed that in rectal cancer, a high N-SUVmax (> 1.5) was more useful in preoperative diagnosis of lymph-node metastasis than was www.nature.com/scientificreports/ the lymph-node diameter 19 . Our study showed similar results in GC, and N-SUVmax was a useful predictor for diagnosing pathological lymph-node metastasis regardless of histological type. Fukagawa et al. reported that it was difficult to accurately diagnose pathological lymph-node metastasis and reported the sensitivity and specificity of CT without FDG-PET/CT were 62.5% and 65.7%, respectively 20 . In our analysis, the accuracy of CT alone in diagnosing pathological lymph-node metastasis was similar to their results, but the combination of diagnosis by FDG-PET/CT improved the accuracy of preoperative diagnosis of pathological lymph-node metastasis. Even in cases diagnosed as positive for lymph-node metastasis by CT, if N-SUVmax was low in FDG-PET/CT, 94% (17/18) of the cases were negative for pathological lymph-node metastasis, which is also useful for diagnosis of negative prediction.
Considering the cost of FDG-PET/CT, it may not be practical to perform it preoperatively for all patients with GC. Therefore, it is important to select patients with comorbidities who are expected to experience more adverse events from NAC treatment, and patients with poor renal function who cannot undergo contrast-enhanced CT. The use of FDG-PET/CT combined with conventional diagnostic methods, such as esophagogastroduodenoscopy and CT, can avoid unnecessary NAC treatment for such patients and contribute to appropriate treatment selection.
This study had several limitations. First, this was a single-institution retrospective study, so selection bias was possible. However, FDG-PET/CT is expensive, and it is valuable to demonstrate the significance of this study in a small number of cases. In addition, because it was performed according to our institution's protocol in general, special pretreatment, such as drinking milk or water to improve the evaluation of GC by FDG-PET/ CT, was not performed.  www.nature.com/scientificreports/ Second, the SUV of small-sized primary tumors and lymph nodes could have been underestimated due to partial volume effects.
Third, in this study, we used SUVmax, which can be easily measured and can also be used to evaluate lymph nodes where volumetric parameters are not available. However, metabolic tumor volume parameters in the evaluation of the primary tumor may have been more useful.
In conclusion, preoperative FDG-PET/CT staging of advanced GC is promising to contribute to the selection of appropriate treatment strategies. Future prospective research is needed to determine how the improved diagnostic accuracy gained by adding FDG-PET/CT to conventional CT scans can lead to improved treatment outcomes.

Patients and methods
Patients. From January 2014 to December 2019, 113 patients who had undergone gastrectomy for adenocarcinoma of GC at Kobe University Hospital were analyzed retrospectively in this study. These patients had been diagnosed with gastric adenocarcinoma by esophagogastroduodenoscopy and had undergone preoperative CT and FDG-PET/CT for initial staging. Preoperative diagnoses of the depth of tumor invasion (T status) and lymph-node metastasis (N status) were diagnosed by esophagogastroduodenoscopy and CT. For evaluation of N status, a size ≥ 8 mm measured by CT was considered positive with reference to the JCOG 1302A study 20 . All patients received distal or total gastrectomy with D1 + or D2 lymph-node dissection. All lymph nodes suspected to be metastatic on preoperative CT or FDG-PET/CT were dissected in surgery. Patients who received any preoperative treatments, such as chemotherapy or radiotherapy, were excluded. The analysis for statistics was divided by 75 years of age, which is the target age group for most of the JCOG clinical trials for NAC in advanced GC. The diagnosis had been confirmed by histopathological examination according to the Japanese Classifica- Table 3. Univariate and multivariate analysis for pathological lymph-node metastasis. OR odds ratio, CI confidence interval, pre preoperative, SUVmax maximum standardized uptake value. *P value < 0.05, **P value < 0.01.

FDG-PET/CT protocol.
Whole-body FDG-PET scans were performed on a PET scanner (Philips Allegro, Philips Medical System, Best, the Netherlands). All patients fasted for ≥ 6 h before injection of 222-333 MBq (6-9 mCi) of FDG to minimize the effects of gastric filling and had blood glucose levels of < 160 mg/dl at the time of injection. Approximately 1 h after IV administration of 222 to 333 MBq (6-9 mCi) of FDG, a static emission scan was performed. Emission PET scans were reconstructed by using the row-action maximum-likelihood algorithm resulting in a 128 × 128 matrix. After the PET scans, the patients underwent CT scans performed at 120 kV and 80 mA. All PET and CT images were integrated by using automatic image-fusion software (Syntegra; SUN Microsystems). The T-SUVmax and N-SUVmax for all patients were measured by FDG-PET/CT (Fig. 2). Two surgeons analyzed all PET images separately under the supervision of one nuclear medicine physicians. When discrepancies were detected, interpretations were achieved via consensus. Pathological results were blinded to the analyst. In detectable lesions, SUVmax was measured from each lesion. In the non-detectable cases, SUVmax was measured from the corresponding site identified by esophagogastroduodenoscopy or enhanced CT scans.
Statistical analyses. Statistical analysis was performed to determine the relationships between clinicopathological parameters, including the SUVmax, and pathological T3/4 (pT3/4) tumor and lymph-node metastasis by using the Mann-Whitney U-test for continuous variables and Fisher's exact test for categorical variables, as appropriate. All P values < 0.05 were considered to be indicative of statistical significance. A multiple logistic regression model was used to identify useful predictors for diagnosis of pT3/T4 tumors and lymph-node metastasis. Covariates found to be significant in the univariate analysis at P < 0.05 were included in the multivariate model. Receiver operating characteristic (ROC) analysis was performed to assess the confidence of the SUVmax for predicting pT3/T4 tumors and lymph-node metastasis, and the areas under the curves (AUCs) were measured. The optimal cutoff value was determined by ROC analysis. All statistical analyses were performed with www.nature.com/scientificreports/ EZR 22 (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, it is a modified version of R commander designed to add statistical functions frequently used in biostatistics.
Human rights statement and informed consent. All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki declaration of 1964 and later versions. Informed consent to be included in the study, or the equivalent, was obtained from all patients.

Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.